Consistent improvements in the resolution of optical lithography techniques have been a key enabler for continuation of Moore's Law. However, as minimum printed feature sizes continue to shrink, the wavelength of light used in modern lithography systems is no longer several times larger than the minimum line dimensions to be printed, e.g., today's 130 nm CMOS processes use 193 nm exposure tools. As a result, modern CMOS processes, for example, are operating in a sub-wavelength lithography regime. The International Technology Roadmap for Semiconductors (ITRS) offers projections on the requirements of next generation lithography systems and states that achieving aggressive microprocessor (MPU) gate lengths and highly controllable gate CD control are two key issues.
To meet these requirements, resolution enhancement techniques (RETs) such as optical proximity correction (OPC) and phase shift mask (PSM) technology are applied to mask design layouts. Advanced mask manufacturing technologies, such as high-precision electron beam machines, high numerical aperture exposure equipment, high-resolution resists, and extreme ultraviolet and possibly electron-beam projection lithography, could also play roles in continued lithography scaling. The result of each of these approaches is a large increase in mask costs.
In the current design-manufacturing interface, no concept of function is injected into the mask flow, i.e., current RETs are oblivious to design intent. Mask writers today work equally hard in perfecting a dummy fill shape, a piece of the company logo, a gate in a critical path, and a gate in a non-critical path, for example. Errors in any of these shapes will trigger rejection of the mask in the inspection tool. The result is unduly low mask throughput and high mask costs.